This invention relates to a device for focussing a lens held in a lens holder. The latter can be moved axially in a lens sleeve supported on a film holder on which it can slide. Devices of this nature are, for example, used in microfilm readers and are therefore commonly known.
Since the lens sleeve is supported on a film holder on which it can slide, the distance between the microfilm and the lens remains constant when traversing the film holder providing the thickness of the upper transparent film holder plate is constant and the microfilm lies level in the film holder. Practice has shown that irregularities in the thickness of the upper plate across its surface often necessitate refocussing the lens when traversing the film holder. This type of refocussing also becomes necessary when the plane of the film holder is not exactly at right angles relative to the lens. In such cases one can observe that the lens sleeve moves up and down when the film holder is traversed. Since the lens sleeve must be vertically movable, it is necessary that movable transmission elements between the focussing wheel and the lens be provided which allow the required vertical movement of the lens sleeve. The provision of movable intermediate transmission elements render the known lens focussing device rather complex. This leads to microfilm readers with two lenses of different focal lengths usually being provided with only one focussing unit that always adjusts both lenses simultaneously and parallel to each other. Hence, switching from one lens to the other invariably necessitates refocussing.